Complex microelectronic devices such as semiconductor chips require numerous connections to other electronic components. Typically, the microelectronic devices are mounted on substrates or external circuit elements, such as printed circuit boards, having electrical contacts, and the contacts on the chip are electrically connected to the contacts of the external circuit element. The external circuit element may have pins or other connectors adapted to accommodate other components, including additional semiconductor chips. Also, the external circuit element may have pins or other connectors adapted to connect the contacts or internal circuitry of the external circuit element to a larger assembly, thereby connecting the chip to the larger assembly.
Connections between microelectronic elements and substrates must meet several demanding and often conflicting requirements. They must provide reliable, low-impedance electrical interconnections. They must also withstand stresses caused by thermal effects during manufacturing processes such as soldering. Other thermal effects occur during operation of the device. As the system operates, it evolves heat and the components of the system, including the chip and the substrate expand. When operation ceases, the components cool and contract. When the assembly is heated and cooled during manufacture or in operation, the chip and the substrate expand and contract at different rates, so that portions of the chip and substrate move relative to one another. Also, the chip and the substrate can warp as they are heated and cooled, causing further movement of the chip relative to the substrate. These and other effects cause repeated strain on electrical elements connecting the chip and the substrate. The interconnection system should withstand repeated thermal cycling without breakage of the electrical connections. The interconnection system should provide a compact assembly, and should be suitable for use with components having closely-spaced contacts. Moreover, the interconnection should be economical.
Various solutions have been proposed to meet these needs. In particular, as disclosed in U.S. Pat. Nos. 5,148,265; 5,148,266; 5,455,390 and in International Publication WO 96/02068, flexible leads may be provided between the contacts on a chip or other microelectronic element and the contact pads of a substrate. According to preferred embodiments taught in these documents, a compliant layer, such as an elastomer or a gel may be provided between the chip and the substrate. Flexible leads connecting the chip and substrate may extend through the compliant layer. In these preferred arrangements, the chip is mechanically decoupled from the substrate, so that the chip and substrate can expand and move independently of one another without excessive stress on the electrical connections between the chip contacts and the contact pads of the substrate. Moreover, the assemblies disclosed in these patents and publications meet the other requirements discussed above. In certain preferred embodiments according to these documents, the chip and the interconnections to the substrate can occupy an area of the substrate about the same size as the chip itself.
Microelectronic elements such as semiconductor chips generate considerable amounts of heat during use. For example, a complex, high-speed chip only a few centimeters square in area may produce tens of watts of heat. This heat must be dissipated to maintain the chip at a safe operating temperature. Improvements in chip mountings and electrical connections, and in related assembly methods, have made it possible to reduce the distance between chips so as to achieve a more compact assembly. Such assemblies, typically referred to as "multichip modules," incorporate one or more substrates with chips disposed close to one another on the substrate. The heat dissipation problems discussed above are particularly extreme in such compact multichip modules.
Considerable effort has been devoted in the art towards meeting these needs for cooling. A general outline of the approaches taken heretofore is set forth in the text Multichip Module Technologies and Alternatives--The Basics, Doane, D. A. and Franzon, P. D., EDS 1993 Van Nostrand Reinhold, New York, N.Y. at chapter 12, pp. 569-613, entitled "Thermal Design Considerations For Multichip Module Applications" (Azar, K., chapter author) and at pages 109-111 of the same reference. As described therein, heat transfer problems in electronic packaging can be addressed in terms of "thermal resistance" of the elements involved. The thermal resistance of any element in the heat transfer path refers to the ratio between the temperature difference across such element and the rate of heat flow through the element. Thermal insulators have high thermal resistance whereas elements which convey heat effectively by conduction or convection have low thermal resistance. The overall thermal resistance of the package is the sum of the individual thermal resistances in series in the heat path between the chip and the ambient environment. The overall thermal resistance in turn provides a ratio between the temperature rise of the chips above ambient temperature and the amount of heat produced in the chips.
As described in the aforementioned reference, the heat conduction pathway may include an element commonly referred to as a "heat sink." There is normally a low thermal resistance connection from the heat sink to the environment. For example, the vanes of the heat sink may be bathed in a flow of forced air or liquid. However, there is generally an appreciable thermal resistance between the semiconductor chip, or other microelectronic components, and the heat sink. Stated another way, it is difficult to provide a low thermal resistance connection between the chip and the heat sink while still meeting all of the other requirements for such a connection. This is because the thermal connection must accommodate relative movement between the chips or other components and the heat sink during use of the device. Such relative movement arises in part from movement of the components and the substrate bearing the components as the assembly undergoes temperature changes during use. When the unit is first supplied with power, the temperature of the chips or other components rises faster than the temperature of the substrate, causing differential thermal expansion, warpage and distortion. Further, the coefficients of thermal expansion of the chips and the substrate normally are not matched with the coefficient of thermal expansion of the heat sink, causing further differential thermal expansion and contraction.
Moreover, the connection between the components and the heat sink should accommodate dimensional tolerances in the components, the substrate and the heat sink itself. For example, the chips themselves may be of different thicknesses. Also, the chips can be supported at different levels above the face of the substrate by solder balls or other mountings. The surfaces of the chips may be tilted from their nominal positions, so that the chip surfaces are out of alignment with the surface of the heat sink. The heat sink itself may not be perfectly flat or parallel to the nominal plane of the chip surfaces. Any elements used to connect the heat sink with the chip or the components should be capable of accommodating these tolerances and misalignments. Considerable efforts have been made in the art heretofore towards satisfying these requirements.
Nonetheless, still further improvement would be desirable. For example, it would be desirable to provide additional connection components and methods which provide effective mechanical decoupling and high resistance to thermally induced stresses, while also providing low cost and high reliability. It would also be desirable to provide a microelectronic package including improved assemblies and methods for dissipating heat therefrom to minimize thermally induced stresses, while also providing high reliability and low cost.